NTSC co-channel interference detectors responsive to received Q-channel signals in digital TV signal receivers

ABSTRACT

An NTSC co-channel interference detector ( 44 ) detects the presence of an interfering NTSC signal in the received Q-channel signal that is orthogonal to the received I-channel signal, rather than detecting the presence of an interfering NTSC signal in the received I-channel signal. By determining whether or not a significant amount of NTSC co-channel interference accompanies the received Q-channel signal, it is inferentially determined whether or not a significant amount of NTSC co-channel interference accompanies the received I-channel signal, such as to cause too many errors in the trellis decoding of equalized received I-channel signal to be corrected by the Reed-Solomon decoder ( 40 ) following the trellis decoder ( 34 ). The accurate determination of co-channel NTSC interference levels is simplified, because essentially no direct bias arises from the quadrature-phase synchronous detection of the pilot carrier of the VSB AM digital television signal.

The present invention relates to digital television systems, and moreparticularly, to circuits employed in the digital television receiverfor determining whether or not there is co-channel interference fromNTSC analog television signals.

BACKGROUND OF THE INVENTION

A Digital Television Standard published Sep. 16, 1995 by the AdvancedTelevision Subcommittee (ATSC) specifies vestigial sideband (VSB)signals for transmitting digital television (DTV) signals in6-MHz-bandwidth television channels such as those currently used inover-the-air broadcasting of National Television Subcommittee (NTSC)analog television signals within the United States. The VSB DTV signalis designed so its spectrum is likely to interleave with the spectrum ofa co-channel interfering NTSC analog TV signal. This is done bypositioning the pilot carrier and the principal amplitude-modulationsideband frequencies of the DTV signal at odd multiples of one-quarterthe horizontal scan line rate of the NTSC analog TV signal that fallbetween the even multiples of one-quarter the horizontal scan line rateof the NTSC analog TV signal, at which even multiples most of the energyof the luminance and chrominance components of a co-channel interferingNTSC analog TV signal will fall. The video carrier of an NTSC analog TVsignal is offset 1.25 MHz from the lower limit frequency of thetelevision channel. The carrier of the DTV signal is offset from suchvideo carrier by 59.75 times the horizontal scan line rate of the NTSCanalog TV signal, to place the carrier of the DTV signal about 309,877.6kHz from the lower limit frequency of the television channel.Accordingly, the carrier of the DTV signal is about 2,690122.4 Hz fromthe middle frequency of the television channel.

The exact symbol rate in the Digital Television Standard is (684/286)times the 4.5 MHz sound carrier offset from video carrier in an NTSCanalog TV signal. The number of symbols per horizontal scan line in anNTSC analog TV signal is 684, and 286 is the factor by which horizontalscan line rate in an NTSC analog TV signal is multiplied to obtain the4.5 MHz sound carrier offset from video carrier in an NTSC analog TVsignal. The symbol rate is 10.762238 megasymbols per second, which canbe contained in a VSB signal extending 5.381119 MHz from DTV signalcarrier. That is, the VSB signal can be limited to a band extending5.690997 MHz from the lower limit frequency of the television channel.

The ATSC standard for digital HDTV signal terrestrial broadcasting inthe United States of America is capable of transmitting either of twohigh-definition television (HDTV) formats with 16:9 aspect ratio. OneHDTV display format uses 1920 samples per scan line and 1080 activehorizontal scan lines per 30 Hz frame with 2:1 field interlace. Theother HDTV display format uses 1280 luminance samples per scan line and720 progressively scanned scan lines of television image per 60 Hzframe. The ATSC standard also accommodates the transmission of DTVdisplay formats other than HDTV display formats, such as the paralleltransmission of four television signals having normal definition incomparison to an NTSC analog television signal.

DTV transmitted by vestigial-sideband (VSB) amplitude modulation (AM)during terrestrial broadcasting in the United States of Americacomprises a succession of consecutive-in-time data fields eachcontaining 313 consecutive-in-time data segments. The data fields may beconsidered to be consecutively numbered modulo-2, with each odd-numbereddata field and the succeeding even-numbered data field forming a dataframe. The frame rate is 20.66 frames per second. Each data segment isof 77.3 microseconds duration. So, with the symbol rate being 10.76 MHzthere are 832 symbols per data segment. Each segment of data begins witha line synchronization code group of four symbols having successivevalues of +S, −S, −S and +S. The value +S is one level below the maximumpositive data excursion, and the value −S is one level above the maximumnegative data excursion. The initial line of each data field includes afield synchronization code group that codes a training signal forchannel-equalization and multipath suppression procedures. The trainingsignal is a 511-sample pseudo-noise sequence (or “PN-sequence”) followedby three 63-sample PN sequences. The middle ones of the 63-sample PNsequences in the field synchronization codes are transmitted inaccordance with a first logic convention in the first line of eachodd-numbered data field and in accordance with a second logic conventionin the first line of each even-numbered data field, the first and secondlogic conventions being one's complementary respective to each other.

The data within data lines are trellis coded using twelve interleavedtrellis codes, each a 2/3 rate trellis code with one uncoded bit. Theinterleaved trellis codes are subjected to Reed-Solomon forwarderror-correction coding, which provides for correction of burst errorsarising from noise sources such as a nearby unshielded automobileignition system. The Reed-Solomon coding results are transmitted as8-level (3 bits/symbol) one-dimensional-constellation symbol coding forover-the-air transmission, which transmissions are made without symbolprecoding separate from the trellis coding procedure. The Reed-Solomoncoding results are transmitted as 16-level (4 bits/symbol)one-dimensional-constellation symbol coding for cablecast, whichtransmissions are made without preceding. The VSB signals have theirnatural carrier wave, which would vary in amplitude depending on thepercentage of modulation, suppressed.

The natural carrier wave is replaced by a pilot carrier wave of fixedamplitude, which amplitude corresponds to a prescribed percentage ofmodulation. This pilot carrier wave of fixed amplitude is generated byintroducing a direct component shift into the modulating voltage appliedto the balanced modulator generating the amplitude-modulation sidebandsthat are supplied to the filter supplying the VSB signal as itsresponse. If the eight levels of 4-bit symbol coding have normalizedvalues of −7, −5, −3, −1, +1, +3, +5 and +7 in the carrier modulatingsignal, the pilot carrier has a normalized value of 1.25. The normalizedvalue of +S is +5, and the normalized value of −S is −5.

In the earlier development of the DVT art it was contemplated that theDTV broadcaster might be called upon to decide whether or not to use asymbol precoder at the transmitter, which symbol precoder would followthe symbol generation circuitry and provide for matched filtering ofsymbols, when used together with a comb filter in each DTV signalreceiver used before the data-slicer in the symbol decoder circuitry asa symbol post-coder. This decision would have depended upon whetherinterference from a co-channel NTSC broadcasting station were expectedor not. Symbol precoding would not have been used for data linesynchronization code groups or during data lines in which data fieldsynchronization data were transmitted. Co-channel interference isreduced at greater distances from the NTSC broadcasting station(s) andis more likely to occur when certain ionospheric conditions obtain, thesummertime months during years of high solar activity being notoriousfor likelihood of co-channel interference. Such interference will notobtain if there are no co-channel NTSC broadcasting stations, of course.If there were likelihood of NTSC interference within his area ofbroadcast coverage, it was presumed that the HDTV broadcaster would usethe symbol precoder to facilitate the HDTV signal being more easilyseparated from NTSC interference; and, accordingly, a comb filter wouldbe employed as symbol post-coder in the DTV signal receiver to completematched filtering. If there were no-possibility of NTSC interference orthere were insubstantial likelihood thereof, in order that flat spectrumnoise would be less likely to cause erroneous decisions as to symbolvalues in the trellis decoder, it was presumed that the DTV broadcasterwould discontinue using the symbol precoder; and, accordingly, thesymbol post-coder would then be disabled in each DTV signal receiver.

U.S. Pat. No. 5,260,793 issued Nov. 9, 1993 to R. W. Citta et alii andentitled “RECEIVER POST CODER SELECTION CIRCUIT” selectively employs apost-coder comb filter for suppressing NTSC interference accompanying areal or in-phase baseband component (I channel) of the complex outputsignal of a demodulator used in a digital high-definition television(HDTV) receiver. The presence of NTSC interference in the I-channelcomponent of the demodulator response is detected for developing controlsignals automatically to enable or disable the comb filter being usedfor suppressing NTSC co-channel interference. During each data fieldsync interval, the input signal to and the output signal from an NTSCsuppression filter of comb filter type in the HDTV signal receiver areeach compared with a respective signal that is known a priori and isdrawn from memory within the HDTV signal receiver. If the minimum resultof comparison with the input signal has less energy than the minimumresult of comparison with the output signal from the NTSC suppressionfilter, this is indicative that the primary cause of variance fromexpected reception is random noise rather than NTSC co-channelinterference. Insofar as the particular digital television receiver isconcerned. reception would be better were precoding and post-coding notemployed in the system, and it is presumed that the broadcaster has notemployed precoding. If the minimum result of comparison with the inputsignal has more energy than the minimum result of comparison with theoutput signal from the NTSC suppression filter, this is indicative thatthe primary cause of variance from expected reception is NTSC co-channelinterference rather than random noise. Insofar as the particular digitaltelevision receiver is concerned, reception would be better werepreceding and post-coding employed in the system, and it is presumedthat the broadcaster has employed precoding.

U.S. Pat. No. 5,546,132 issued Aug. 13, 1996 to K. S. Kim et alii andentitled “NTSC INTERFERENCE DETECTOR” describes the use of post-codercomb filtering for suppressing co-channel NTSC interference when thepresence of such interference is detected in NTSC-extraction comb filterresponse to the I channel. U.S. Pat. No. 5,546,132 does not specificallydescribe an imaginary or quadrature-phase baseband component (Q channel)of a complex output signal being supplied from the demodulator used in adigital HDTV signal receiver. A digital HDTV signal receiver thatsynchrodynes the VSB AM signals to baseband commonly employs ademodulator that includes an in-phase synchronous detector for supplyingreceived I-channel signal for trellis decoding (after post-coding, ifprecoding is used at the transmitter) and further includes aquadrature-phase synchronous detector for supplying received Q-channelsignal. The received Q-channel signal is lowpass filtered to generate anautomatic frequency and phase control (AFPC) signal for the localoscillator supplying carrier for synchrodyning. The specification anddrawing of U.S. Pat. No. 5,479,449 issued Dec. 26, 1996 to C. B. Pateland A. L. R. Limberg, entitled “DIGITAL VSB DETECTOR WITH BANDPASS PHASETRACKER, AS FOR INCLUSION IN AN HDTV RECEIVER”, and assigned to SamsungElectronics Co., Ltd., is incorporated herein by reference. The reader'sattention is specifically directed to elements 22-27 in FIG. 1 of thedrawing of U.S. Pat. No. 5,479,449 and the description thereof in theaccompanying specification. These elements are used in the describedHDTV signal receiver for carrying out complex demodulation of the VSB AMfinal intermediate-frequency signal. U.S. Pat. No. 5,479,449 describescomplex demodulation of the VSB AM final I-F signal being carried out inthe digital regime. but in alternative digital TV receiver designscomplex demodulation of the VSB AM final I-F signal is instead carriedout in the analog regime.

In both U.S. Pat. Nos. 5,260,793 and 5,546,132 post-coding is enabledduring times of substantial co-channel NTSC interference and otherwisedisabled, with the control signal for such selective enablement beingdeveloped from the received I-channel signal. The determination ofco-channel NTSC interference levels is complicated by the direct biasaccompanying the co-channel NTSC interference, which direct bias arisesfrom the in-phase synchronous detection of the pilot carrier of the VSBAM DTV signal. This is particularly a problem in DTV signal receivers inwhich automatic gain control does not tightly regulate the amplitude ofthe received I-channel signal recovered by in-phase synchronousdetection.

The video carrier of an NTSC signal is 1.25 MHz from edge of the6-MHz-wide broadcast channel, while the carrier for a DTV signal forterrestrial through-the-air broadcast is 310 kHz from edge of the6-MHz-wide broadcast channel. A co-channel NTSC signal does not exhibitsymmetrical amplitude-modulation sidebands with respect to the carrierof the vestigial-sideband amplitude-modulation (VSB AM) carrying digitalinformation. Accordingly, artifacts of the NTSC video carrier at 940 kHzremove from DTV signal carrier and artifacts of its sidebands are notwell canceled in the DTV signal as synchrodyned to baseband. Nor, ofcourse, are artifacts of the NTSC audio carrier and its sidebands, theNTSC audio carrier being at 5.44 MHz remove from DTV signal carrier.

The Digital Television Standard the ATSC published Sep. 16, 1995 doesnot allow for the use of precoding of all data at the DTV transmitter tocompensate for post-coding incidental to subsequent use of combfiltering in a DTV signal receiver to reject NTSC co-channelinterference. Instead, only the initial symbol in the trellis decodingis precoded. This procedure by itself does not facilitate a DTV signalreceiver using comb filtering to reject NTSC co-channel interferencebefore data slicing procedures are undertaken. A DTV signal receiverthat does not reject artifacts of NTSC co-channel interference beforedata slicing procedures are undertaken will not have good receptionunder strong NTSC co-channel interference conditions as may be caused bythe DTV signal receiver being remote from the DTV transmitter or havingan analog TV transmitter very closeby. In the DTV signal as synchrodynedto baseband the artifacts of the video carrier of a co-channelinterfering NTSC color TV signal are at 59.75f_(H), f_(H) being thehorizontal scan frequency of that signal. The artifact of the colorsubcarrier is at 287.25f_(H), and the artifact of the unmodulated NTSCaudio carrier is at 345.75f_(H). Comb filtering procedures are notentirely satisfactory for suppressing artifacts of thefrequency-modulated NTSC audio carrier, particularly under conditions offrequency modulation in which carrier frequency deviation is large,since correlation (or anti-correlation) of samples of the FM carrier attimes separated by any substantial fixed delay may not be particularlygood, the inventor points out. The inventor recommends that thefiltering used to establish the overall bandwidth ofintermediate-frequency amplification be such as to reject the FM audiocarrier of any co-channel interfering NTSC analog TV signal. Combfiltering procedures are more satisfactory for separating the basebandDTV signal from the artifacts of the NTSC video carrier, the low videofrequencies, and the chrominance signal frequencies close to the colorcarrier. This is because these artifacts tend to exhibit goodcorrelation between samples separated by certain specific delayintervals and to exhibit good anti-correlation between samples separatedby certain other specific delay intervals.

In U.S. patent application Ser. No. 08/746,520 filed by the inventor onNov. 12, 1996 and entitled “DTV RECEIVER WITH FILTER IN I-F CIRCUITRY TOSUPPRESS FM SOUND CARRIER OF NTSC CO-CHANNEL INTERFERING SIGNAL”, theinventor advocates preceding data-slicing in a DTV signal receiver withcomb filtering to suppress NTSC co-channel interference when thatinterference is sufficiently large as to affect data-slicing adversely.The inventor teaches how to compensate in the symbol decoding procedurefor the effects of such comb filtering upon symbol coding when it isselectively done. It is, then, still useful to be able to determine whenNTSC co-channel interference is larger than a prescribed valuedenominated as being acceptably small, so that this determination can beused for controlling the selective use of comb filtering to suppressNTSC co-channel interference.

NTSC co-channel interference will appear in the imaginary orquadrature-phase baseband component (Q channel) of the complex outputsignal of a demodulator used in a DTV signal receiver whenever NTSCco-channel interference appears in the real or in-phase basebandcomponent (I channel) of that complex output signal. Accordingly, anNTSC interference detector can be arranged so that its NTSC extractingfilter responds to the received Q-channel signal, rather than thereceived I-channel signal. By determining whether or not a significantamount of NTSC co-channel interference accompanies the receivedQ-channel signal, it is inferentially determined whether or not asignificant amount of NTSC co-channel interference accompanies thereceived I-channel signal, such as to cause too many errors in thetrellis decoding of equalized received I-channel signal to be correctedby the Reed-Solomon decoder following the trellis decoder. The accuratedetermination of co-channel NTSC interference levels is simplified.because essentially no direct bias arises from the quadrature-phasesynchronous detection of the pilot carrier of the VSB AM DTV signal.

SUMMARY OF THE INVENTION

A method for processing vestigial-sideband amplitude-modulated digitaltelevision signals in a digital television signal receiver in accordancewith an aspect of the invention comprises the following steps. A complexdemodulation of vestigial-sideband amplitude-modulated digitaltelevision signals susceptible to co-channel NTSC interference isperformed, to separate a received l-channel baseband signal and areceived Q-channel baseband signal in an orthogonal relationship withsaid received I-channel baseband signal. Then, it is estimated whetherartifacts of co-channel NTSC interference accompanying the receivedI-channel baseband signal are of significant level by determiningwhether further artifacts of co-channel NTSC interference accompanyingthe received Q-channel baseband signal exceed a prescribed level.

A method for determining, in accordance with an aspect of the invention,whether or not a digital television receiver is to employ comb filteringto suppress co-channel NTSC interference before trellis decodingcomprises the following steps. A complex demodulation of digitaltelevision signals is performed to separate a received I-channelbaseband signal and a received Q-channel baseband signal in anorthogonal relationship with the received I-channel baseband signal.Whether or not artifacts of co-channel NTSC interference that are ofsignificant level accompany the received Q-channel baseband signal isdetermined. If no artifacts of co-channel NTSC interference ofsignificant level are determined to accompany the received Q-channelbaseband signal, the received I-channel baseband signal is symboldecoded without being comb filtered to generate decoded symbols fortrellis decoding. If artifacts of co-channel NTSC interference ofsignificant level are determined to accompany the received Q-channelbaseband signal, the received I-channel baseband signal is comb filteredto generate comb-filtered I-channel baseband signal in which co-channelNTSC interference is suppressed, symbol decoding is performed on thecomb-filtered I-channel baseband signal; and the result of symboldecoding responsive to the comb-filtered I-channel baseband signal ispostcoded to generate decoded symbols for trellis decoding.

NTSC co-channel interference detectors embodying the invention invarious of its aspects detect the presence of an interfering NTSC signalin the Q channel that is orthogonal to the I channel. Adaptive NTSCco-channel interference suppression circuitry embodying the invention infurther of its aspects uses these NTSC co-channel interference detectorsfor controlling whether comb filtering is to be performed forsuppressing NTSC co-channel interference in the I channel before dataslicing in a digital television receiver.

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1 is a block diagram of a portion of a digital television receiverthat includes a symbol decoder with NTSC co-channel interferencesuppression circuitry which, in accordance with the invention, isselectively activated depending on the response of an NTSC co-channelinterference detector responsive to the Q-channel signal from a complexdemodulator for DTV signal.

FIG. 2 is a block diagram of an NTSC co-channel interference detectorconstructed in accordance with the invention to respond to the Q-channelsignal from a complex demodulator for DTV signal.

FIG. 3 is a flow chart of operation in a portion of the FIG. 1 digitaltelevision receiver showing how equalization procedures are modifieddepending on whether or not comb filtering to suppress co-channel NTSCinterference is employed.

FIG. 4 is a block schematic diagram showing details of a portion of theFIG. 1 digital television signal receiver when the NTSC-rejection combfilter employs a 12-symbol delay.

FIG. 5 is a block schematic diagram showing details of the FIG. 2 NTSCco-channel interference detector when a 12-symbol delay is employedtherewithin.

FIG. 6 is a block schematic diagram showing details of a portion of theFIG. 1 DTV signal receiver when the NTSC-rejection comb filter employs a6-symbol delay.

FIG. 7 is a block schematic diagram showing details of the FIG. 2 NTSCco-channel interference detector when a 6-symbol delay is employedtherewithin.

FIG. 8 is a block schematic diagram showing details of a portion of theFIG. 1 DTV signal receiver when the NTSC-rejection comb filter employs a2-video-line delay.

FIG. 9 is a block schematic diagram showing details of the FIG. 2 NTSCco-channel interference detector when a 2-video-line delay is employedtherewithin.

FIG. 10 is a block schematic diagram showing details of a portion of theFIG. 1 DTV signal receiver when the NTSC-rejection comb filter employs a262-video-line delay.

FIG. 11 is a block schematic diagram showing details of the FIG. 2 NTSCco-channel interference detector when a 262-video-line delay is employedtherewithin.

FIG. 12 is a block schematic diagram showing details of a portion of theFIG. 1 DTV signal receiver when the NTSC-rejection comb filter employs a2-video-frame delay.

FIG. 13 is a block schematic diagram showing details of the FIG. 2 NTSCco-channel interference detector when a 2-video-frame delay is employedtherewithin.

Each of FIGS. 14 and 15 is a block schematic diagram showing details ofa respective alternative type of NTSC co-channel interference detectorthat can be employed in the FIG. 1 DTV signal receiver.

FIG. 16 is a block schematic diagram of a digital television receiverembodying the invention, in which DTV signal receiver a plurality ofcomb filters and associated NTSC co-channel interference detectors areemployed for selectively filtering against artifacts of NTSC co-channelinterference.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

At various points in the circuits shown in the FIGURES of the drawing,shimming delays have to be inserted in order that the sequence ofoperation is correct, as will be understood by those skilled inelectronic design. Unless there is something out of the ordinary about aparticular shimming delay requirement, it will not be explicitlyreferred to in the specification that follows.

FIG. 1 shows a digital television signal receiver used for recoveringerror-corrected data, which data are suitable for recording by a digitalvideo cassette recorder (DVCR) or for MPEG-2 decoding and display in atelevision set. The FIG. 1 DTV signal receiver is shown as receivingtelevision broadcast signals from a receiving antenna 8, but can receivethe signals from a cable network instead. The television broadcastsignals are supplied as input signal to “front end” electronics 10. The“front end” electronics 10 generally include a radio-frequency amplifierand first detector for converting radio-frequency television signals tointermediate-frequency television signals, supplied as input signal toan intermediate-frequency (IF) amplifier chain 12 for vestigial-sidebandDTV signals. The DTV signal receiver is preferably of plural-conversiontype with the IF amplifier chain 12 including an IF amplifier foramplifying DTV signals as converted to an ultra-high-frequency band bythe first detector, a second detector for converting the amplified DTVsignals to a very-high-frequency band, and a further IF amplifier foramplifying DTV signals as converted to the VHF band. If demodulation tobaseband is performed in the digital regime, the IF amplifier chain 12will further include a third detector for converting the amplified DTVsignals to a final intermediate-frequency band closer to baseband.

Preferably, a surface-acoustic-wave (SAW) filter is used in the IFamplifier for the UHF band, to shape channel selection response andreject adjacent channels. This SAW filter cuts off rapidly just beyond5.38 MHz remove from the suppressed carrier frequency of the VSB DTVsignal and the pilot carrier, which is of like frequency and of fixedamplitude. This SAW filter accordingly rejects much of thefrequency-modulated sound carrier of any co-channel interfering analogTV signal. Removing the FM sound carrier of any co-channel interferinganalog TV signal in the IF amplifier chain 12 prevents artifacts of thatcarrier being generated when the final I-F signal is detected to recoverbaseband symbols and forestalls such artifacts interfering withdata-slicing of those baseband symbols during symbol decoding. Theprevention of such artifacts interfering with data-slicing of thosebaseband symbols during symbol decoding is better than can beaccomplished by relying on comb-filtering before data-slicing,particularly if the differential delay in the comb filter is more than afew symbol epochs.

The final IF output signals from the IF amplifier chain 12 are suppliedto a complex demodulator 14, which demodulates the vestigial-sidebandamplitude-modulation DTV signal in the final intermediate-frequency bandto recover a real baseband signal and an imaginary baseband signal.Demodulation may be done in the digital regime after analog-to-digitalconversion of a final intermediate-frequency band in the few megacyclerange as described in U. S. Pat. No. 5,479,449, for example.Alternatively, demodulation may be done in the analog regime, in whichcase the results are usually subjected to analog-to-digital conversionto facilitate further processing. The complex demodulation is preferablydone by in-phase (I) synchronous demodulation and quadrature-phase (Q)synchronous demodulation. The digital results of the foregoingdemodulation procedures conventionally have 8-bit accuracy or more anddescribe 2N-level symbols that encode N bits of data. Currently, 2N iseight in the case where the FIG. 1 DTV signal receiver receives athrough-the-air broadcast via the antenna 12 and is sixteen in the casewhere the FIG. 1 DTV signal receiver receives cablecast. The concern ofthe invention is with the reception of terrestrial through-the-airbroadcasts, and FIG. 1 does not show the portions of the DTV signalreceiver providing symbol decoding and error-correction decoding forreceived cablecast transmissions.

Symbol synchronization and equalization circuitry 16 receives at leastthe digitized real samples of the in-phase (I-channel) baseband signalfrom the complex demodulator 14; in the FIG. 1 DTV signal receiver thecircuitry 16 is shown also receiving the digitized imaginary samples ofthe quadrature-phase (Q-channel) baseband signal. The circuitry 16includes a digital filter with adjustable weighting coefficients thatcompensates for ghosts and tilt in the received signal. The symbolsynchronization and equalization circuitry 16 provides symbolsynchronization or “de-rotation” as well as amplitude equalization andghost removal. Symbol synchronization and equalization circuitry inwhich symbol synchronization is accomplished before amplitudeequalization is known from U.S. Pat. No. 5,479,449. In such designs thedemodulator 14 will supply oversampled demodulator response containingreal and imaginary baseband signals to the symbol synchronization andequalization circuitry 16. After symbol synchronization, the oversampleddata are decimated to extract baseband I-channel signal at normal symbolrate, to reduce sample rate through the digital filtering used foramplitude equalization and ghost removal. Symbol synchronization andequalization circuitry in which amplitude equalization precedes symbolsynchronization, “de-rotation” or “phase tracking” is also known tothose skilled in the art of digital signal receiver design.

Each sample of the circuitry 16 output signal is resolved to ten or morebits and is, in effect, a digital description of an analog symbolexhibiting one of (2N=8) levels. The circuitry 16 output signal iscarefully gain-controlled by any one of several known methods, so theideal step levels for symbols are known. One method of gain control,preferred because the speed of response of such gain control isexceptionally rapid, regulates the direct component of the real basebandsignal supplied from the complex demodulator 14 to a normalized level of+1.25. This method of gain control is generally described in U.S. Pat.No. 5,479,449 and is more specifically described by C. B. Patel et aliiin U.S. Pat. No. 5,573,454 issued Jun. 3, 1997, entitled “AUTOMATIC GAINCONTROL OF RADIO RECEIVER FOR RECEIVING DIGITAL HIGH-DEFINITIONTELEVISION SIGNALS”, and incorporated herein by reference.

The output signal from the circuitry 16 is supplied as input signal todata sync detection circuitry 18, which recovers data fieldsynchronization information F and data segment synchronizationinformation S from the equalized baseband I-channel signal.Alternatively, the input signal to data sync detection circuitry 18 canbe obtained prior to equalization.

The equalized I-channel signal samples at normal symbol rate supplied asoutput signal from the circuitry 16 arc applied as the input signal toan NTSC-rejection comb filter 20. The comb filter 20 includes a firstdelay device 201 to generate a pair of differentially delayed streams ofthe 2N-level symbols and a first linear combiner 202 for linearlycombining the differentially delayed symbol streams to generate the combfilter 20 response. As described in U.S. Pat. No. 5,260,793, the firstdelay device 201 can provide a delay equal to the period of twelve2N-level symbols, and the first linear combiner 202 can be a subtractor.Each sample of the comb filter 20 output signal is resolved to ten ormore bits and is, in effect, a digital description of an analog symbolexhibiting one of (4N−1)=15 levels.

The symbol synchronization and equalization circuitry 16 is presumed bedesigned to suppress the direct bias component of its input signal (asexpressed in digital samples), which direct bias component has anormalized level of +1.25 and appears in the real baseband signalsupplied from the complex demodulator 14 owing to detection of the pilotcarrier. Accordingly, each sample of the circuitry 16 output signalapplied as comb filter 20 input signal is, in effect, a digitaldescription of an analog symbol exhibiting one of the followingnormalized levels: −7, −5, −3, −1, +1, +3, +5 and +7. These symbollevels are denominated as “odd” symbol levels and are detected by anodd-level data-slicer 22 to generate interim symbol decoding results of000, 001, 010, 011, 100, 101, 110 and 111, respectively.

Each sample of the comb filter 20 output signal is, in effect, a digitaldescription of an analog symbol exhibiting one of the followingnormalized levels: −14, −12, −10, −8, −6, −4, −2, 0, +2, +4, +6, +8,+10, +12 and +14. These symbol levels are denominated as “even” symbollevels and are detected by an even-level data-slicer 24 to generateprecoded symbol decoding results of 001, 010, 011, 100, 101, 110, 111,000, 001, 010, 011, 100, 101, 110, and 111, respectively.

The data-slicers 22 and 24 can be of the so-called “hard decision” type,as presumed up to this point in the description, or can be of theso-called “soft decision” type used in implementing a Viterbi decodingscheme. Arrangements are possible in which the odd-level data-slicer 22and the even-level data-slicer 24 are replaced by a single data-slicer,using multiplexer connections to shift its place in circuit and toprovide bias to modify its slicing ranges, but these arrangements arenot preferred because of the complexity of operation.

The symbol synchronization and equalization circuitry 16 is presumed inthe foregoing description to be designed to suppress the direct biascomponent of its input signal (as expressed in digital samples), whichdirect bias component has a normalized level of +1.25 and appears in thereal baseband signal supplied from the complex demodulator 14 owing todetection of the pilot carrier. Alternatively, the symbolsynchronization and equalization circuitry 16 is designed to preservethe direct bias component of its input signal, which simplifies thedesign of the equalization filter in the circuitry 16 somewhat. In suchcase the data-slicing levels in the odd-level data-slicer 22 are offsetto take into account the direct bias component accompanying the datasteps in its input signal. Providing that the first linear combiner 202is a subtractor, whether the circuitry 16 is designed to suppress or topreserve the direct bias component of its input signal has noconsequence in regard to the data-slicing levels in the even-leveldata-slicer 24. However, if the differential delay provided by the firstdelay device 201 is chosen so that the first linear combiner 202 is anadder, the data-slicing levels in the even-level data-slicer 24 shouldbe offset to take into account the doubled direct bias componentaccompanying the data steps in its input signal.

A comb filter 26 is used after the data-slicers 22 and 24 to generate apostcoding filter response to the precoding filter response of the combfilter 20. The comb filter 26 includes a 3-input multiplexer 261, asecond linear combiner 262, and a second delay device 263 with delayequal to that of the first delay device 201 in the comb filter 20. Thesecond linear combiner 262 is a modulo-8 adder if the first linearcombiner 202 is a subtractor and is a modulo-8 subtractor if the firstlinear combiner 202 is an adder. The first linear combiner 202 and thesecond linear combiner 262 may be constructed as respective read-onlymemories (ROMs) to speed up linear combination operations sufficientlyto support the sample rates involved. The output signal from themultiplexer 261 furnishes the response from the postcoding comb filter26 and is delayed by the second delay device 263. The second linearcombiner 262 combines precoded symbol decoding results from theeven-level data-slicer 24 with the output signal from the second delaydevice 263.

The output signal of the multiplexer 261 reproduces one of the threeinput signals applied to the multiplexer 261, as selected in response tofirst, second and third states of a multiplexer control signal suppliedto the multiplexer 261 from a controller 28. The first input port of themultiplexer 261 receives ideal symbol decoding results supplied frommemory within the controller 28 during times when data fieldsynchronization information F and data segment synchronizationinformation S from the equalized baseband I-channel signal are recoveredby the data sync detection circuitry 18. The controller 28 supplies thefirst state of the multiplexer control signal to the multiplexer 261during these times, conditioning the multiplexer 261 to furnish, as thefinal coding results which arc its output signal, the ideal symboldecoding results supplied from memory within the controller 28. Theodd-level data-slicer 22 supplies interim symbol decoding results as itsoutput signal to the second input port of the multiplexer 261. Themultiplexer 261 is conditioned by the second state of the multiplexercontrol signal to reproduce the interim symbol decoding results in thefinal coding results supplied from the multiplexer 261. The secondlinear combiner 262 supplies postcoded symbol decoding results as itsoutput signal to the third input port of the multiplexer 261. Themultiplexer 261 is conditioned by the third state of the multiplexercontrol signal to reproduce the postcoded symbol decoding results forexample. Running errors in the postcoded symbol decoding results fromthe postcoding comb filter 26 are curtailed by feeding back the idealsymbol decoding results supplied from memory within the controller 28during times data sync detection circuitry 18 recovers data fieldsynchronization information F and data segment synchronizationinformation S.

The output signal from the multiplexer 261 in the postcoding comb filter26 comprises the final symbol decoding results in 3-parallel-bit groups,assembled by a data assembler 30 for application to a data interleaver32. The data interleaver 32 commutates the assembled data into paralleldata streams for application to trellis decoder circuitry 34. Trellisdecoder circuitry 34 conventionally uses twelve trellis decoders. Thetrellis decoding results are supplied from the trellis decoder circuitry34 to data de-interleaver circuitry 36 for de-commutation. Byte parsingcircuitry 38 converts the data interleaver 36 output signal into bytesof Reed-Solomon error-correction coding for application to Reed-Solomondecoder circuitry 40, which performs Reed-Solomon decoding to generatean error-corrected byte stream supplied to a data de-randomizer 42. Thedata de-randomizer 42 supplies reproduced data to the remainder of thereceiver (not shown). The remainder of a complete DTV signal receiverwill include a packet sorter, an audio decoder, an MPEG-2 decoder and soforth. The remainder of a DTV signal receiver incorporated in a digitaltape recorder/reproducer will include circuitry for converting the datato a form for recording.

An NTSC co-channel interference detector 44 supplies the controller 28with an indication of whether NTSC co-channel interference is ofsufficient strength as to cause uncorrectable error in the data-slicingperformed by the data-slicer 22. If detector 44 indicates the NTSCco-channel interference is not of such strength, the controller 28 willsupply the second state of multiplexer control signal to the multiplexer261 at times other than those times when data field synchronizationinformation F and data segment synchronization information S arerecovered by the data sync detection circuitry 18. This conditions themultiplexer 261 to reproduce as its output signal the interim symboldecoding results supplied from the odd-level data-slicer 22. If detector44 indicates the NTSC co-channel interference is of sufficient strengthto cause uncorrectable error in the data-slicing performed by thedata-slicer 22, the controller 28 will supply the third state ofmultiplexer control signal to the multiplexer 261 at times other thanthose times when data field synchronization information F and datasegment synchronization information S are recovered by the data syncdetection circuitry 18. This conditions the multiplexer 261 to reproduceas its output signal the postcoded symbol decoding results provided assecond linear combining results from the second linear combiner 262.

The invention disclosed in this specification and its accompanyingdrawing is characterized by the NTSC co-channel interference detector 44responding to the artifacts of NTSC co-channel interference that appearin the Q-channel output signal of the complex demodulator 14 for DTVsignal. The detector 44 can be connected to detect the artifacts of NTSCco-channel interference in Q-channel output signal from the complexdemodulator 14 extracted before the symbol synchronization andequalization circuitry 16, but FIG. 1 shows the detector 44 connected todetect the artifacts in Q-channel output signal extracted from theresponse of the symbol synchronization and equalization circuitry 16.

FIG. 2 shows a form the NTSC co-channel interference detector 44 cantake in one embodiment of the invention. The Q-channel output signalextracted from the response of the symbol synchronization andequalization circuitry 16 is supplied to a node 440, either directly orafter filtering by a bandwidth selection filter 441 that supplies to thenode 440 a response to those portions of Q-channel output signal morelikely to contain artifacts of NTSC co-channel interference. The signalat node 440 is applied as input signal to a third delay device 442 to besubjected to a third delay. A third linear combiner 443 linearlycombines the signal at node 440 with that signal as delayed by the thirddelay device 442 to generate a comb filter response in which artifactsof NTSC co-channel interference are rejected. A fourth linear combiner444 linearly combines the signal at node 440 with that signal as delayedby the third delay device 442 to generate a comb filter response inwhich artifacts of NTSC co-channel interference are selected. One of thethird and fourth linear combiners is a digital adder and the other is adigital subtractor, the choice of which is which depending on the delayby the third delay device 442 is designed to provide. The amplitude ofthe comb filter response from the third linear combiner 443 is detectedby an amplitude detector 445, the amplitude of the comb filter responsefrom the fourth linear combiner 444 is detected by an amplitude detector446, and the results of amplitude detection by the amplitude detectors445 and 446 are compared by an amplitude comparator 447. The amplitudecomparator 447 supplies an output bit indicative of whether or not theresponse of the amplitude detector 446 substantially exceeds theresponse of the amplitude detector 445. This output bit is used forselecting between the second and third states of multiplexer 261operation. For example, this output bit from the amplitude comparator447 can be one of two control bits the controller 28 supplies to themultiplexer 261 in the postcoding comb filter 26 of the FIG. 1, theother control bit being indicative of whether or not signal suppliedfrom the controller 28 is to be reproduced in the multiplexer 261response.

The amplitude detectors 445 and 446 can, by way of example, be envelopedetectors with a time constant equal to several data sample intervals,so that differences in the data components of their input signals tendto average out to low value supposing them to be random. Differences inrandom noise accompanying the responses of the linear combiners 443 and444 tend to average out to zero as well. Accordingly, when the amplitudecomparator 447 for comparing the amplitude detection responses ofamplitude detectors 445 and 446 indicates those responses differ morethan a prescribed amount, this is indicative that artifacts of anyco-channel interfering analog television signal are above a significantlevel for the Q-channel baseband signal. This significant level for theQ-channel baseband signal corresponds to the significant level for theI-channel baseband signal. Errors in symbol decoding done by simply dataslicing the I-channel baseband signal are correctable by the trellis andReed-Solomon error-correction coding as long as artifacts of anyco-channel interfering analog television signal are kept below thesignificant level for the I-channel baseband signal.

When the amplitude of the comb filter response from the fourth linearcombiner 444 in which artifacts of NTSC co-channel interference areselected is substantially larger than the amplitude of the comb filterresponse from the third linear combiner 443 in which artifacts of NTSCco-channel interference are rejected, this difference can then bepresumed to be caused by the presence of artifacts of NTSC co-channelinterference in the signal at node 440. The output bit supplied by theamplitude comparator 447 for this condition conditions the multiplexer261 not to be operable in its second state, thereby deselecting theinterim symbol decoding results from the odd-level data slicer 22 fromappearing as final symbol decoding results from the multiplexer 261.

When the amplitude of the comb filter response from the fourth linearcombiner 444 in which artifacts of NTSC co-channel interference areselected is not substantially larger than the amplitude of the combfilter response from the third linear combiner 443 in which artifacts ofNTSC co-channel interference are rejected, this lack of difference canbe presumed to indicate the absence of artifacts of NTSC co-channelinterference in the signal at node 440. The output bit supplied by theamplitude comparator 447 for this condition conditions the multiplexer261 not to be operable in its third state, thereby deselecting thepostcoded symbol decoding results from the second linear combiner 262from appearing as final symbol decoding results from the multiplexer261.

The inclusion of the bandwidth selection filter 441 may be unnecessaryor even undesirable, depending on the length of delay in the third delayelement 442 and on the design of the amplitude detectors 445 and 446.Instead of being envelope detectors, the amplitude detectors 445 and 446may detect the energy of departures of their input signals from symbolcode levels as inferred from pilot carrier strength; the bandwidthselection filter 441 would not be used in such case. If the length ofdelay in the third delay element 442 is such that artifacts of the NTSCsound carrier tend not to cancel very well, but the artifacts of theNTSC video carrier and color subcarrier tend to cancel reasonably well,and if the amplitude detectors 445 and 446 are envelope detectors, thenthe bandwidth selection filter 441 can take the form of afinite-impulse-response (FIR) digital lowpass filter 4410 with a cut-offfrequency no higher than about 5.4 MHz, as shown in FIGS. 7, 9, 11, 13and 14.

FIG. 3 is a flow chart showing how equalization procedures are modifiedin the FIG. 1 DTV signal receiver depending on whether or not combfiltering to suppress co-channel NTSC interference is employed. Theinventor points out that the presence of the artifacts of co-channelNTSC interference in the baseband symbol coding introduces errors intothe calculation of equalization filter kernel coefficients unlessspecial measures are taken in the calculations to negate theseartifacts.

In an initial step S1, a complex demodulation of digital televisionsignals is continuously performed by the complex demodulator 14 in theFIG. 1 DTV signal receiver, to separate a received I-channel basebandsignal and a received Q-channel baseband signal in an orthogonalrelationship with the received I-channel baseband signal. In a decisionstep S2, which is also continuously performed by the NTSC co-channelinterference detector 44 in the FIG. 1 DTV signal receiver, it isdetermined whether or not a significant amount of co-channel NTSCinterference accompanies the received Q-channel baseband signal.

A significant amount of co-channel NTSC interference in a DTV signalreceiver is that level which causes the number of errors incurred duringtrellis decoding to significantly degrade the error correctingcapabilities of the two-dimensional Reed-Solomon decoding that followstrellis decoding, causing substantial numbers of bit errors in theultimately recovered data, under conditions of normally noisy reception.The significant amount of co-channel NTSC interference in a DTV signalreceiver of particular design is readily determined by experiments on aprototype thereof.

If in the decision step S2 no significant amount of co-channel NTSCinterference is determined to accompany the received Q-channel basebandsignal, a step S3 of adjusting the kernel weights of a digitalequalization filter, in order to equalize its response to the I-channelbaseband signal, and a subsequent step S4 of symbol decoding theequalization filter response resulting from the step S3 are performed togenerate symbol decoding result used in a step S5 of trellis decodingthe symbol decoding result to correct errors therein. The step S5 oftrellis decoding is followed by a step S6 of Reed-Solomon decoding tocorrect errors in the result of trellis decoding and a step S7 ofdeformatting the result of Reed-Solomon decoding.

If in the decision step S2 a significant amount of co-channel NTSCinterference is determined to accompany the received Q-channel basebandsignal, a step S8 of comb filtering the received I-channel basebandsignal to generate comb-filtered I-channel baseband signal is performedusing a suitable comb filter. In a step S9 the kernel weights of thedigital equalization filter are adjusted to conform the response of thecascaded digital equalization filter and comb filter to an idealresponse for such filter cascade. A step S10 of symbol decoding theresponse of such filter cascade is performed and thereafter a step 11 ofpostcoding the symbol decoding response is performed to obtain correctedsymbol decoding result to be used in the step S5 of trellis decoding.The step S5 of trellis decoding is still followed by the step S6 ofReed-Solomon decoding to correct errors in the result of trellisdecoding and the step S7 of deformatting the result of Reed-Solomondecoding.

The submethod used for adjusting the kernel weights of the digitalequalization filter in step S3 of equalizing digital equalization filterresponse is similar to the adjustment of the kernel weights of thedigital equalization filter used in the prior art. Adjustment can bemade by calculating the discrete Fourier transform (DFT) of the receiveddata field synchronization code or a prescribed portion thereof anddividing it by the DFT of the ideal data field synchronization code orprescribed portion thereof to determine the DFT of the DTV transmissionchannel. The DFT of the DTV transmission channel is normalized withrespect to the largest term(s) to characterize the channel, and thekernel weights of the digital equalization filter are selected tocomplement the normalized DFT characterizing the channel. This method ofadjustment is described in greater detail by C. B. Patel et alii in U.S.Pat. No. 5,331,416 issued Jul. 19, 1994 and entitled “METHODS FOROPERATING GHOST-CANCELATION CIRCUITRY FOR TV RECEIVER OR VIDEORECORDER”, for example. This method is preferable for initial adjustmentof the kernel weights of the digital equalization filter because theinitial adjustment is more rapidly made than by using adaptiveequalization. After initial adjustment of the kernel weights of thedigital equalization filter, adaptive equalization methods arepreferred. A block LMS method for carrying out adaptive equalization isdescribed by J. Yang et alii in U.S. Pat. No. 5,648,987 issued Jul. 15,1997 and entitled “RAPID-UPDATE ADAPTIVE CHANNEL-EQUALIZATION FILTERINGFOR DIGITAL RADIO RECEIVERS, SUCH AS HDTV RECEIVERS”. A continuous LMSmethod for carrying out adaptive equalization is described by A. L. R.Limberg in U. S. patent application Ser. No. 08/832,674 filed Apr. 4,1997 and entitled “DYNAMICALLY ADAPTIVE EQUALIZER SYSTEM AND METHOD”.

In the step S9 the submethod by which the kernel weights of the digitalequalization filter are adjusted to conform the response of the cascadeddigital equalization filter and comb filter to an ideal response forsuch filter cascade can be carried out using DFT, especially whenperforming rapid initial equalization prior to switching to adaptiveequalization. Adjustment is made by calculating the discrete Fouriertransform (DFT) of the received data field synchronization code or aprescribed portion thereof, as comb filtered by the comb filter 20 forrejecting NTSC artifacts and dividing it by the DFT of the ideal datafield synchronization code or prescribed portion thereof, as so combfiltered, to determine the DFT of the DTV transmission channel. The DFTof the DTV transmission channel is then normalized with respect to thelargest term(s) to characterize the channel, and the kernel weights ofthe digital equalization filter are adjusted to complement thenormalized DFT characterizing the channel. After initial adjustment ofthe kernel weights of the digital equalization filter, adaptiveequalization methods are preferably employed. These adaptiveequalization methods differ from those used when artifacts of NTSCco-channel interference are insignificant in that the number of possiblevalid signal states is doubled, less one, by using the comb filter 20for rejecting NTSC artifacts.

FIG. 4 is a block schematic diagram showing details of a portion of theFIG. 1 DTV signal receiver using a species 120 of the NTSC-rejectioncomb filter 20 and a species 126 of the postcoding comb filter 26. Asubtractor 1202 serves as the first linear combiner in theNTSC-rejection comb filter 120, and a modulo-8 adder 1262 serves as thesecond linear combiner in the postcoding comb filter 126. TheNTSC-rejection comb filter 120 uses a first delay device 1201 exhibitinga delay of twelve symbol epochs, and the postcoding comb filter 126 usesa second delay device 1263 also exhibiting a delay of twelve symbolepochs. The 12-symbol delay exhibited by each of the delay devices 1201and 1263 is close to one cycle delay of the artifact of the analog TVvideo carrier at 59.75 times the analog TV horizontal scan frequencyf_(H). The 12-symbol delay is close to five cycles of the artifact ofthe analog TV chrominance subcarrier at 287.25 times f_(H). The12-symbol delay is close to six cycles of the artifact of the analog TVsound carrier at 345.75 times f_(H). This is the reason that thedifferentially combined response of the subtractor 1202 to the audiocarrier, to the video carrier and to frequencies close to chrominancesubcarrier differentially delayed by the first delay device 1201 tendsto have reduced co-channel interference. However, in portions of a videosignal in which edges cross a horizontal scan line, the amount ofcorrelation in the analog TV video signal at such distances in thehorizontal spatial direction is quite low.

A species 1261 of the multiplexer 261 is controlled by a multiplexercontrol signal that is in its second state most of the time when it isdetermined there is insufficient NTSC co-channel interference to causeuncorrectable error in the output signal from the data-slicer 22 andthat is in its third state most of the time when it is determined thereis sufficient NTSC co-channel interference to cause uncorrectable errorin the output signal from the data-slicer 22. The multiplexer 1261 isconditioned by its control signal being in its third state to feed backthe modulo-8 sum results of the adder 1262, as delayed twelve symbolepochs by the delay device 1263, to the adder 1262 as a summand. This isa modular accumulation procedure in which a single error propagates as arunning error, with error recurring every twelve symbol epochs. Runningerrors in the postcoded symbol decoding results from the postcoding combfilter 126 are curtailed by the multiplexer 1261 being placed into itsfirst state for four symbol epochs at the beginning of each datasegment, as well as during the entirety of each data segment containingfield sync. When this control signal is in its first state, themultiplexer 1261 reproduces as its output signal ideal symbol decodingresults supplied from memory in the controller 28. The introduction ofideal symbol decoding results into the multiplexer 1261 output signalhalts a running error. Since there are 4+69(12) symbols per datasegment, the ideal symbol decoding results slip back four symbol epochsin phase each data segment, so no running error can persist for longerthan three data segments.

FIG. 5 is a block schematic diagram showing details of a species 144 ofthe FIG. 2 NTSC co-channel interference detector 44 with a third delayelement 1442 therewithin providing a 12-symbol delay of Q-channel signalfrom the symbol synchronization and equalization circuitry 16 applieddirectly to the node 440. The third linear combiner is a digitalsubtractor 1443 differentially combining differentially-delayedQ-channel signal from the symbol synchronization and equalizationcircuitry 16 to generate the comb filter response supplied to theamplitude detector 445 in which response artifacts of NTSC co-channelinterference are rejected. The fourth linear combiner is a digital adder1444 additively combining the differentially-delayed Q-channel signal togenerate the comb filter response supplied to the amplitude detector 445in which response artifacts of NTSC co-channel interference areselected. This NTSC co-channel interference detector 144 is especiallywell suited for use in the FIG. 1 DTV signal receiver when it uses thespecies 120 of the NTSC-rejection comb filter 20 and the species 126 ofthe postcoding comb filter 26. Since the comb filtering employing thesubtractor 1443 rejects artifacts arising from NTSC audio carrier, fromNTSC video carrier and from NTSC color subcarrier the bandwidthselection filter 441 is unnecessary before the node 440.

FIG. 6 is a block schematic diagram showing details of a portion of theFIG. 1 DTV signal receiver using a species 220 of the NTSC-rejectioncomb filter 20 and a species 226 of the postcoding comb filter 26. TheNTSC-rejection comb filter 220 uses a first delay device 2201 exhibitinga delay of six symbol epochs, and the postcoding comb filter 226 uses asecond delay device 2263 also exhibiting a delay of six symbol epochs.The 6-symbol delay exhibited by each of the delay devices 2201 and 2263is close to 0.5 cycle delay of the artifact of the analog TV videocarrier at 59.75 times the analog TV horizontal scan frequency f_(H),close to 2.5 cycles of the artifact of the analog TV chrominancesubcarrier at 287.25 times f_(H), and close to 3 cycles of any artifactof the analog TV audio carrier at 345.75 times f_(H). An adder 2202serves as the first linear combiner in the NTSC-rejection comb filter220, and a modulo-8 subtractor 2262 serves as the second linear combinerin the postcoding comb filter 226. Since the delay exhibited by thedelay devices 2201 and 2263 is shorter than the delay exhibited by thedelay devices 1201 and 1263, although nulls near frequencies convertedfrom analog TV carrier frequencies are narrower band, there is morelikely to be good anti-correlation in the signals additively combined bythe adder 2202 than there is likely to be good correlation in thesignals differentially combined by the subtractor 1202. The suppressionof the sound carrier is poorer in the NTSC-rejection comb filter 220response than in the NTSC-rejection comb filter 120 response. However,if the sound carrier of a co-channel interfering analog TV signal hasbeen suppressed by SAW filtering or a sound trap in the IF amplifierchain 12, the poor sound rejection of the comb filter 220 is not aproblem. The responses to sync tips is reduced in duration using theNTSC-rejection comb filter 220 of FIG. 6 rather than the NTSC-rejectioncomb filter 120 of FIG. 4, so there is substantially reduced tendency tooverwhelm error-correction in the trellis decoding and Reed-Solomoncoding.

A species 2261 of the multiplexer 261 is controlled by a multiplexercontrol signal that is in its second state most of the time when it isdetermined there is insufficient NTSC co-channel interference to causeuncorrectable error in the output signal from the data-slicer 22 andthat is in its third state most of the time when it is determined thereis sufficient NTSC co-channel interference to cause uncorrectable errorin the output signal from the data-slicer 22. The multiplexer 2261 isconditioned by its control signal being in its third state to feed backthe modulo-8 sum results of the adder 2262, as delayed six symbol epochsby the delay device 2263, to the adder 2262 as a summand. This is amodular accumulation procedure in which a single error propagates as arunning error, with error recurring every six symbol epochs. Runningerrors in the postcoded symbol decoding results from the postcoding combfilter 226 are curtailed by the multiplexer 2261 being placed into itsfirst state for four symbol epochs at the beginning of each datasegment, as well as during the entirety of each data segment containingfield sync. When this control signal is in its first state, themultiplexer 2261 reproduces as its output signal ideal symbol decodingresults supplied from memory in the controller 28. The introduction ofideal symbol decoding results into the multiplexer 2261 output signalhalts a running error. Since there are 4+138(6) symbols per datasegment, the ideal symbol decoding results slip back four symbol epochsin phase each data segment, so no running error can persist for longerthan two data segments. The likelihood of a protracted period of runningerror in the postcoding comb filter 226 is substantially less than inthe postcoding comb filter 126, although the running error recurs morefrequently and affects twice as many of the twelve interleaved trelliscodes.

FIG. 7 is a block schematic diagram showing details of a species 244 ofthe FIG. 2 NTSC co-channel interference detector 44 with a third delayelement 2442 therewithin providing a 6-symbol delay to Q-channel signalapplied to the node 440. The third linear combiner is a digital adder2443 additively combining the differentially-delayed Q-channel signal togenerate the comb filter response supplied to the amplitude detector 445in which response artifacts of NTSC co-channel interference arerejected. The fourth linear combiner is a digital subtractor 2444differentially combining differentially-delayed Q-channel signal fromthe symbol synchronization and equalization circuitry 16 to generate thecomb filter response supplied to the amplitude detector 446 in whichresponse artifacts of NTSC co-channel interference are selected. ThisNTSC co-channel interference detector 244 is especially well suited foruse in the FIG. 1 DTV signal receiver when it uses the species 220 ofthe NTSC-rejection comb filter 20 and the species 226 of the postcodingcomb filter 26.

FIG. 8 is a block schematic diagram showing details of a portion of theFIG. 1 DTV signal receiver using a species 320 of the NTSC-rejectioncomb filter 20 and a species 326 of the postcoding comb filter 26. TheNTSC-rejection comb filter 320 uses a first delay device 3201 exhibitinga delay of 1368 symbol epochs, which delay is substantially equal to theepoch of two horizontal scan lines of an analog TV signal, and thepostcoding comb filter 326 uses a second delay device 3263 alsoexhibiting such delay. The first linear combiner in the NTSC-rejectioncomb filter 320 is an adder 3202, and the second linear combiner in thepostcoding comb filter 326 is a modulo-8 subtractor 3262.

A species 3261 of the multiplexer 261 is controlled by a multiplexercontrol signal that is in its second state most of the time when it isdetermined there is insufficient NTSC co-channel interference to causeuncorrectable error in the output signal from the data-slicer 22 andthat is in its third state most of the time when it is determined thereis sufficient NTSC co-channel interference to cause uncorrectable errorin the output signal from the data-slicer 22. The DTV signal receiverpreferably contains circuitry for detecting change between alternatescan lines in the NTSC co-channel interference, so that the controller28 can withhold supplying the third state of the multiplexer 3261control signal under such conditions.

The multiplexer 3261 is conditioned by its control signal being in itsthird state to feed back the modulo-8 sum results of the adder 3262, asdelayed 1368 symbol epochs by the delay device 3263, to the adder 3262as a summand. This is a modular accumulation procedure in which a singleerror propagates as a running error, with error recurring every 1368symbol epochs. This symbol code span is longer than the span for asingle block of the Reed-Solomon code, so a single running error isreadily corrected during Reed-Solomon decoding. Running errors in thepostcoded symbol decoding results from the postcoding comb filter 326are curtailed by the multiplexer 3261 being placed into its first stateduring the entirety of each data segment containing field sync, as wellas for four symbol epochs at the beginning of each data segment. Whenthis control signal is in its first state, the multiplexer 3261reproduces as its output signal ideal symbol decoding results suppliedfrom memory in the controller 28. The introduction of ideal symboldecoding results into the multiplexer 3261 output signal halts a runningerror. The 16.67 millisecond duration of an NTSC video field exhibitsphase slippage against the 24.19 millisecond duration of a DTV datafield, so the DTV data segments containing field sync eventually scanthe entire NTSC frame raster. The 525 lines in the NTSC frame rastereach contain 684 symbol epochs, for a total of 359,100 symbol epochs.Since this is somewhat less than 432 times the 832 symbol epochs in aDTV data segment containing field sync, one can guess with reasonableconfidence that running errors of duration longer than 432 data fieldswill be expunged by the multiplexer 3261 reproducing ideal symboldecoding results during DTV data segments containing field sync. Thereis also phase slippage between data segments, for the start code groupsof which ideal symbol decoding results are available, and NTSC videoscan lines. One can estimate 359,100 symbol epochs, which is 89,775times the four symbol epochs in a code start group, are scanned during89,775 consecutive data segments. Since there are 313 data segments perDTV data field, one can guess with reasonable confidence that runningerrors of duration longer than 287 data fields will be expunged by themultiplexer 3261 reproducing ideal symbol decoding results during thecode start groups. The two sources of suppression of running errors arereasonably independent of each other, so running errors of durationlonger than two hundred or so data fields are quite unlikely.Furthermore, if NTSC co-channel interference dips low at a time when therunning error recurs, to condition the multiplexer 3261 for reproducingthe response of the data-slicer 22 as its output signal, the error maybe corrected earlier than would otherwise be the case.

The FIG. 8 NTSC-rejection comb filter 320 is quite good in suppressingdemodulation artifacts generated in response to analog TV horizontalsynchronizing pulses, as well as suppressing many of the demodulationartifacts generated in response to analog TV vertical synchronizingpulses and equalizing pulses. These artifacts are the co-channelinterference with highest energy. Except where there isscan-line-to-scan-line change in the video content of the-analog TVsignal over the period of two scan lines, the NTSC-rejection comb filter320 provides reasonably good suppression of that video contentregardless of its color. The suppression of the FM audio carrier of theanalog TV signal is reasonably good, in case it has not been suppressedby a tracking rejection filter in the symbol synchronization andequalization circuitry 16. Artifacts of most analog TV color bursts aresuppressed in the NTSC-rejection comb filter 320 response, too.Furthermore, the filtering provided by the NTSC-rejection comb filter320 is “orthogonal” to the NTSC-interference rejection built into thetrellis decoding procedures.

FIG. 9 is a block schematic diagram showing details of a species 344 ofthe FIG. 2 NTSC co-channel interference detector 44 with a third delayelement 3442 therewithin providing a 2-video-line delay of 1368 symbolepochs to Q-channel signal applied to the node 440. The third linearcombiner is a digital adder 3443 additively combining thedifferentially-delayed Q-channel signal to generate the comb filterresponse supplied to the amplitude detector 445 in which responseartifacts of NTSC co-channel interference are rejected. The fourthlinear combiner is a digital subtractor 3444 differentially combiningdifferentially-delayed Q-channel signal from the symbol synchronizationand equalization circuitry 16 to generate the comb filter responsesupplied to the amplitude detector 446 in which response artifacts ofNTSC co-channel interference are selected. This NTSC co-channelinterference detector 344 is especially well suited for use in the FIG.1 DTV signal receiver when it uses the species 320 of the NTSC-rejectioncomb filter 20 and the species 326 of the postcoding comb filter 26.

FIG. 10 is a block schematic diagram showing details of a portion of theFIG. 1 DTV signal receiver using a species 420 of the NTSC-rejectioncomb filter 20 and a species 426 of the postcoding comb filter 26. TheNTSC-rejection comb filter 420 uses a first delay device 4201 exhibitinga delay of 179,208 symbol epochs, which delay is substantially equal tothe period of 262 horizontal scanning lines of an analog TV signal, andthe postcoding comb filter 426 uses a second delay device 4261 alsoexhibiting such delay. An adder 4202 serves as the first linear combinerin the NTSC-rejection comb filter 420, and a modulo-8 subtractor 4262serves as the second linear combiner in the postcoding comb filter 426.

A species 4261 of the multiplexer 261 is controlled by a multiplexercontrol signal that is in its second state most of the time when it isdetermined there is insufficient NTSC co-channel interference to causeuncorrectable error in the output signal from the data-slicer 22 andthat is in its third state most of the time when it is determined thereis sufficient NTSC co-channel interference to cause uncorrectable errorin the output signal from the data-slicer 22. The DTV signal receiverpreferably contains circuitry for detecting field-to-field change in theNTSC co-channel interference, so that the controller 28 can withholdsupplying the third state of the multiplexer 4261 control signal undersuch conditions.

The multiplexer 4261 is conditioned by its control signal being in itsthird state to feed back the modulo-8 sum results of the adder 4262, asdelayed 179,208 symbol epochs by the delay device 4263, to the adder4262 as a summand. This is a modular accumulation procedure in which asingle error propagates as a running error, with error recurring every179,208 symbol epochs. This symbol code span is longer than the span fora single block of the Reed-Solomon code, so a single running error isreadily corrected during Reed-Solomon decoding. Running errors in thepostcoded symbol decoding results from the postcoding comb filter 426are curtailed by the multiplexer 4261 being placed into its first stateduring the entirety of each data segment containing field sync, as wellas for four symbol epochs at the beginning of each data segment. Whenthis control signal is in its first state, the multiplexer 4261reproduces as its output signal ideal symbol decoding results suppliedfrom memory in the controller 28. The introduction of ideal symboldecoding results into the multiplexer 4261 output signal halts a runningerror. The maximum number of data fields required to expunge runningerror in the multiplexer 4261 output signal is presumably substantiallythe same as required to expunge running error in the multiplexer 3261output signal. However, the number of times the error recurs in thatperiod is lower by a factor of 131.

The FIG. 10 NTSC-rejection comb filter 420 suppresses most demodulationartifacts generated in response to analog TV vertical synchronizingpulses and equalizing pulses, as well as suppressing all thedemodulation artifacts generated in response to analog TV horizontalsynchronizing pulses. These artifacts are the co-channel interferencewith highest energy. Also, the NTSC-rejection comb filter 420 suppressesartifacts arising from the video content of the analog TV signal thatdoes not change from field to field or line-to-line, getting rid ofstationary patterns irrespective of their horizontal spatial frequencyor color. Artifacts of most analog TV color bursts are suppressed in theNTSC-rejection comb filter 420 response, too.

FIG. 11 is a block schematic diagram showing details of a species 444 ofthe FIG. 2 NTSC co-channel interference detector 44 with a third delayelement 4442 therewithin providing a 262-video-line delay of 179,208symbol epochs to Q-channel signal applied to the node 440. The thirdlinear combiner is a digital adder 4443 additively combining thedifferentially-delayed Q-channel signal to generate the comb filterresponse supplied to the amplitude detector 445 in which responseartifacts of NTSC co-channel interference are rejected. The fourthlinear combiner is a digital subtractor 4444 differentially combiningdifferentially-delayed Q-channel signal from the symbol synchronizationand equalization circuitry 16 to generate the comb filter responsesupplied to the amplitude detector 446 in which response artifacts ofNTSC co-channel interference are selected. This NTSC co-channelinterference detector 444 is especially well suited for use in the FIG.1 DTV signal receiver when it uses the species 420 of the NTSC-rejectioncomb filter 20 and the species 426 of the postcoding comb filter 26.

FIG. 12 is a block schematic diagram showing details of a portion of theFIG. 1 DTV signal receiver using a species 520 of the NTSC-rejectioncomb filter 20 and a species 526 of the postcoding comb filter 26. TheNTSC-rejection comb filter 520 uses a first delay device 5201 exhibitinga delay of 718.200 symbol epochs, which delay is substantially equal tothe period of two frames of an analog TV signal, and the postcoding combfilter 526 uses a second delay device 5261 also exhibiting such delay. Asubtractor 5202 serves as the first linear combiner in theNTSC-rejection comb filter 520, and a modulo-8 adder 5262 serves as thesecond linear combiner in the postcoding comb filter 526.

A species 5261 of the multiplexer 261 is controlled by a multiplexercontrol signal that is in its second state most of the time when it isdetermined there is insufficient NTSC co-channel interference to causeuncorrectable error in the output signal from the data-slicer 22 andthat is in its third state most of the time when it is determined thereis sufficient NTSC co-channel interference to cause uncorrectable errorin the output signal from the data-slicer 22. The DTV signal receiverpreferably contains circuitry for detecting change between alternateframes in the NTSC co-channel interference, so that the controller 28can withhold supplying the third state of the multiplexer 5261 controlsignal under such conditions.

The multiplexer 5261 is conditioned by its control signal being in itsthird state to feed back the modulo-8 sum results of the adder 5262, asdelayed 718,200 symbol epochs by the delay device 5263, to the adder5262 as a summand. This is a modular accumulation procedure in which asingle error propagates as a running error, with error recurring every718,200 symbol epochs. This symbol code span is longer than the span fora single block of the Reed-Solomon code, so a single running error isreadily corrected during Reed-Solomon decoding. Running errors in thepostcoded symbol decoding results from the postcoding comb filter 526are curtailed by the multiplexer 5261 being placed into its first stateduring the entirety of each data segment containing field sync, as wellas for four symbol epochs at the beginning of each data segment. Whenthis control signal is in its first state, the multiplexer 5261reproduces as its output signal ideal symbol decoding results suppliedfrom memory in the controller 28. The introduction of ideal symboldecoding results into the multiplexer 5261 output signal halts a runningerror. The maximum number of data fields required to expunge runningerror in the multiplexer 5261 output signal is presumably substantiallythe same as required to expunge running error in the multiplexer 3261output signal. However, the number of times the error recurs in thatperiod is lower by a factor of 525.

The FIG. 12 NTSC-rejection comb filter 520 suppresses all demodulationartifacts generated in response to analog TV vertical synchronizingpulses and equalizing pulses, as well as suppressing all thedemodulation artifacts generated in response to analog TV horizontalsynchronizing pulses. These artifacts are the co-channel interferencewith highest energy. Also, the NTSC-rejection comb filter 520 suppressesartifacts arising from the video content of the analog TV signal thatdoes not change over two frames, getting rid of such very stationarypatterns irrespective of their spatial frequency or color. Artifacts ofall analog TV color bursts are suppressed in the NTSC-rejection combfilter 520 response, too.

FIG. 13 is a block schematic diagram showing details of a species 544 ofthe FIG. 2 NTSC co-channel interference detector 44 with a third delayelement 5442 therewithin providing a 2-video-frame delay of 718,200symbol epochs to Q-channel signal applied to the node 440. The thirdlinear combiner is a digital adder 5443 additively combining thedifferentially-delayed Q-channel signal to generate the comb filterresponse supplied to the amplitude detector 445 in which responseartifacts of NTSC co-channel interference are rejected. The fourthlinear combiner is a digital subtractor 5444 differentially combiningdifferentially-delayed Q-channel signal from the symbol synchronizationand equalization circuitry 16 to generate the comb filter responsesupplied to the amplitude detector 446 in which response artifacts ofNTSC co-channel interference are selected. This NTSC co-channelinterference detector 544 is especially well suited for use in the FIG.1 DTV signal receiver when it uses the species 520 of the NTSC-rejectioncomb filter 20 and the species 526 of the postcoding comb filter 26.

One skilled in the art of television system design will discern otherproperties of correlation and anti-correlation in analog TV signals thatcan be exploited in the design of NTSC-rejection filters of still othertypes than those shown in FIGS. 4, 6, 8, 10 and 12. The use ofNTSC-rejection filters that cascade two NTSC-rejection filters of thetypes already disclosed increases the 2N levels of the baseband signalsto (8N−1) data levels, which increases the difficulty of data slicingduring symbol decoding. Such filters may be required to overcomeparticularly bad co-channel interference problems despite theirshortcoming of reducing signal-to-noise for random noise interferencewith symbol decoding. Cascading filters to improve NTSC rejection andselection in the co-channel interference detector has less drawbackassociated with it.

FIG. 14 illustrates cascading of filters to improve NTSC rejection andselection in a species 644 of the co-channel interference detector 44that can be considered to be a modification of the FIG. 13 co-channelinterference detector 544. The response of the FIR digital lowpassfilter 4410 to equalized Q-channel signal from the symbolsynchronization and equalization circuitry 16 is applied to the node440. There is a saving in hardware if the comb filter section requiringthe longest delay is earliest in the cascade, since the same longestdelay element can be used both by the NTSC-reject comb filter and by theNTSC-select comb filter. As in the FIG. 13 co-channel interferencedetector 544, the signal at the node 440 is supplied to the2-video-frame delay element 5442 in the FIG. 14 co-channel interferencedetector 644; and the resulting differentially delayed signals areadditively combined by the adder 5443 and differentially combined by thesubtractor 5444.

The sum response from the adder 5443 is subjected to furtherNTSC-rejection filtering to generate the NTSC-rejection comb filterresponse supplied to the amplitude detector 445. More particularly, adelay device 6441 provides 6-symbol differential delay to the sumresponse from the adder 5443, and the differentially delayed sumresponses from the adder 5443 are additively combined by a digital adder6442 to generate the NTSC-rejection comb filter response supplied to theamplitude detector 445.

The difference response from the subtractor 5444 is subjected to furtherNTSC-rejection filtering to generate the NTSC-selection comb filterresponse supplied to the amplitude detector 446. More particularly, adelay device 6443 provides 6-symbol differential delay to the differenceresponse from the subtractor 5444, and the differentially delayeddifference responses from the subtractor 5444 are differentiallycombined by a digital subtractor 6444 to generate the NTSC-selectioncomb filter response supplied to the amplitude detector 446. The resultsof amplitude detection by the amplitude detectors 445 and 446 arecompared by an amplitude comparator 447. The amplitude comparator 447supplies an output bit indicative of whether or not the response of theamplitude detector 446 substantially exceeds the response of theamplitude detector 445. This output bit is used for selecting betweenthe second and third states of multiplexer 261 operation in the FIG. 1DTV signal receiver which uses the co-channel interference detector 644as its co-channel interference detector 44. The cascade filtering in theco-channel interference detector 644 utilizes the temporal combfiltering between alternative NTSC video frames to suppress NTSCartifacts arising from synchronizing information and static videocomponents. The cascade filtering in the co-channel interferencedetector 644 utilizes intraframe spatial comb filtering to suppress NTSCartifacts arising from dynamic video components.

FIG. 15 further illustrates cascading of filters to improve NTSCrejection and selection in another species 744 of the co-channelinterference detector 44 that can be considered to be a modification ofthe FIG. 13 co-channel interference detector 544. The equalizedQ-channel signal from the symbol synchronization and equalizationcircuitry 16 is applied directly to the node 440, the FIR digitallowpass filter 4410 being unnecessary since the second stage of combfiltering suppresses artifacts of the audio carrier of any co-channelinterfering NTSC analog TV signal. A s in the FIG. 13 co-channelinterference detector 544, the signal at the node 440 is supplied to the2-video-frame delay element 5442 in the FIG. 15 co-channel interferencedetector 644; and the resulting differentially delayed signals areadditively combined by the adder 5443 and differentially combined by thesubtractor 5444.

The sum response from the adder 5443 is subjected to furtherNTSC-rejection filtering to generate the NTSC-rejection comb filterresponse supplied to the amplitude detector 445. More particularly, adelay device 7441 provides 12-symbol differential delay to the sumresponse from the adder 5443, and the differentially delayed sumresponses from the adder 5443 are differentially combined by a digitalsubtractor 7442 to generate the NTSC-rejection comb filter responsesupplied to the amplitude detector 445.

The difference response from the subtractor 5444 is subjected to furtherNTSC-rejection filtering to generate the NTSC-selection comb filterresponse supplied to the amplitude detector 446. More particularly, adelay device 7443 provides 12-symbol differential delay to thedifference response from the subtractor 5444, and the differentiallydelayed difference responses from the subtractor 5444 are additivelycombined by a digital adder 7444 to generate the NTSC-selection combfilter response supplied to the amplitude detector 446. The results ofamplitude detection by the amplitude detectors 445 and 446 are comparedby an amplitude comparator 447. The amplitude comparator 447 supplies anoutput bit indicative of whether or not the response of the amplitudedetector 446 substantially exceeds the response of the amplitudedetector 445. This output bit is used for selecting between the secondand third states of multiplexer 261 operation in the FIG. 1 DTV signalreceiver which uses the co-channel interference detector 744 as itsco-channel interference detector 44. The cascade filtering in theco-channel interference detector 744 utilizes the temporal combfiltering between alternative NTSC video frames to suppress NTSCartifacts arising from synchronizing information and static videocomponents. The cascade filtering in the co-channel interferencedetector 744 utilizes intraframe spatial comb filtering to suppress NTSCartifacts arising from audio components and dynamic video components.

FIG. 16 shows modification of the FIG. 1 DTV signal receiver as thusfardescribed, constructed in accordance with a further aspect of theinvention so as to utilize a plurality of parallelly operated even-leveldata slicers A24, B24 and C24, each preceded by a respectiveNTSC-rejection comb filter and succeeded by a respective postcoding combfilter. The even-level data-slicer A24 converts the response of anNTSC-rejection filter A20 of a first type to first precoded symboldecoding results for application to a postcoding comb filter A26 of afirst type. The even-level data-slicer B24 converts the response of anNTSC-rejection filter B20 of a second type to second precoded symboldecoding results for application to a postcoding comb filter B26 of asecond type. The even-level data-slicer C24 converts the response of anNTSC-rejection filter C20 of a third type to third precoded symboldecoding results for application to a postcoding comb filter C26 of athird type. The odd-level data-slicer 22 supplies interim symboldecoding results to the postcoding comb filters A26, B26 and C26. Theprefixes A, B and C in the identification numbers for the elements ofFIG. 15 are different integers which will correspond to respective onesof the integers 1, 2, 3, 4 and 5 when receiver portions as shown in onesof FIGS. 4, 6, 8, 10 and 12 are employed.

A co-channel interference detector A44 of a first type determines fromthe Q-channel signal how effective the NTSC-rejection filter A20 offirst type will be in reducing co-channel interference from an analog TVsignal in the current equalized I-channel signal. A co-channelinterference detector B44 of a second type determines from the Q-channelsignal how effective the NTSC-rejection filter B20 of second type willbe in reducing co-channel interference from an analog TV signal in thecurrent equalized I-channel signal. A co-channel interference detectorC44 of a third type determines from the Q-channel signal how effectivethe NTSC-rejection filter C20 of third type will be in reducingco-channel interference from an analog TV signal in the currentequalized I-channel signal. The suppression of the pilot carrier in theQ-channel signal facilitates the co-channel interference detectors A44,B44 and C44 providing indications of the relative effectiveness of theNTSC-rejection comb filters A20, B20 and C20.

Symbol decoding selection circuitry 90 generates a best estimate ofcorrect symbol decoding for application to the data assembler 30. Thisbest estimate is generated by selecting among ideal symbol decodingresults from the controller 28, interim symbol decoding results from theodd-level data slicer 22, and postcoded symbol decoding results from thepostcoding comb filters A26, B26 and C26. The symbol decoding selectioncircuitry 90 responds to indications of effectiveness from theco-channel interference detectors A44, B44 and C44 to formulate thisbest estimate unless the controller 28 supplies further symbol selectioninformation to the symbol decoding selection circuitry 90. The furthersymbol selection information supplied from the controller 28 includesindications of when synchronizing codes occur, which indicationscondition the best estimate to be made based on ideal symbol decodingresults from the controller 28. The best estimate of symbol decodingresults is used to correct the summation procedures in the matching combfilters A26, B26 and C26 in preferred embodiments of the FIG. 16 DTVsignal receiver.

If the co-channel interference detectors A44, B44 and C44 all indicatelack of substantial artifacts from NTSC co-channel interference at timesother than when synchronizing codes occur, the symbol decoding selectioncircuitry 90 responds to select the interim symbol decoding results fromthe odd-level data slicer 22 as the best estimate of correct symboldecoding results. This minimizes the effect of Johnson noise on symboldecoding.

If at least one of the co-channel interference detectors A44, B44 andC44 indicates substantial artifacts from NTSC co-channel interference attimes other than when synchronizing codes occur, the symbol decodingselection circuitry 90 responds to select the postcoded symbol decodingresults from the postcoding comb filter A26, B26 or C26 following theone of the NTSC-rejection comb filters A20, B20 and C20 that bestsuppresses artifacts from NTSC co-channel interference as determined bythe co-channel interference detectors A44, B44 and C44.

The high-energy demodulation artifacts generated in response to analogTV synchronizing pulses, equalizing pulses, and color bursts are allsuppressed when the NTSC-rejection comb filter A20 additively combinesalternate video frames. Also, artifacts arising from the video contentof the analog TV signal that does not change over two frames aresuppressed, getting rid of stationary patterns irrespective of theirspatial frequency or color. The co-channel interference detector A44 ofFIG. 13 is used together with the FIG. 12 symbol decoding circuitry.

The remaining problem of suppressing demodulation artifacts primarilyconcerns suppressing those demodulation artifacts arising fromframe-to-frame difference at certain pixel locations within the analogTV signal raster. These demodulation artifacts can be suppressed byintra-frame filtering techniques. The NTSC-rejection comb filter B20 andthe postcoding comb filter B26 circuitry can be chosen to suppressremnant demodulation artifacts by relying on correlation in thehorizontal direction, and the NTSC-rejection comb filter C20 and thepostcoding comb filter C26 circuitry can be chosen to suppress remnantdemodulation artifacts by relying on correlation in the verticaldirection. Consider how such a design decision can be furtherimplemented.

If the sound carrier of a co-channel interfering analog TV signal is notsuppressed by SAW filtering or a sound trap in the IF amplifier chain12, the NTSC-rejection comb filter B20 and the postcoding comb filterB26 circuitry are advantageously chosen to be of types like theNTSC-rejection comb filter 120 and the postcoding comb filter 126circuitry of FIG. 4. The co-channel interference detector B44 of FIG. 5is used together with the FIG. 4 symbol decoding circuitry.

If the sound carrier of a co-channel interfering analog TV signal issuppressed by SAW filtering or a sound trap in the IF amplifier chain12, the NTSC-rejection comb filter B20 and the postcoding comb filterB26 circuitry are advantageously chosen to be of types like theNTSC-rejection comb filter 220 and the postcoding comb filter 226circuitry of FIG. 6. This is because the anti-correlation between videocomponents only six symbol epochs away from each other is usually betterthan the correlation between video components twelve symbol epochs awayfrom each other. The co-channel interference detector B44 of FIG. 7 isused together with the FIG. 6 symbol decoding circuitry.

The optimal choice of the NTSC-rejection comb filter C20 and thepostcoding comb filter C26 circuitry is less straightforward, because ofthe choice one must make (in consideration of field interlace in theinterfering analog TV signal) whether to choose the temporally closerscan line in the same field or the spatially closer line in thepreceding field to be combined with the current scan line in theNTSC-rejection comb filter C20. Choosing the temporally closer scan linein the same field is generally the better choice, since jump cutsbetween fields are less likely to ravage NTSC rejection by the combfilter C20. With such choice, the NTSC-rejection comb filter C20 and thepostcoding comb filter C26 circuitry are of types like theNTSC-rejection comb filter 320 and the postcoding comb filter 326circuitry of FIG. 8. The co-channel interference detector C44 of FIG. 9is used together with the FIG. 8 symbol decoding circuitry.

With the other choice instead, the NTSC-rejection comb filter C20 andthe postcoding comb filter C26 circuitry are of types like theNTSC-rejection comb filter 420 and the postcoding comb filter 426circuitry of FIG. 10. The co-channel interference detector C44 of FIG.11 is used together with the FIG. 10 symbol decoding circuitry.

What is claimed is:
 1. A method for processing vestigial-sidebandamplitude-modulated digital television signals in a digital televisionsignal receiver, said method comprising steps of: performing a complexdemodulation of vestigial-sideband amplitude-modulated digitaltelevision signals susceptible to co-channel NTSC interference, toseparate a received I-channel baseband signal and a received Q-channelbaseband signal in an orthogonal relationship with said receivedI-channel baseband signal; and estimating whether artifacts ofco-channel NTSC interference accompanying said received I-channelbaseband signal are of significant level by determining whether furtherartifacts of co-channel NTSC interference accompanying said receivedQ-channel baseband signal exceed a prescribed level.
 2. A method fordetermining whether or not comb filtering to suppress co-channel NTSCinterference is to be employed before trellis decoding in a digitaltelevision signal receiver, said method comprising steps of: performinga complex demodulation of vestigial-sideband amplitude-modulated digitaltelevision signals to separate a received I-channel baseband signal anda received Q-channel baseband signal in an orthogonal relationship withsaid received I-channel baseband signal; determining whether or notartifacts of co-channel NTSC interference of significant level accompanysaid received Q-channel baseband signal; if said artifacts of co-channelNTSC interference of significant level are determined not to accompanysaid received Q-channel baseband signal, symbol decoding responsive tosaid received I-channel baseband signal without comb filtering thereofto generate decoded symbols for said trellis decoding; if said artifactsof co-channel NTSC interference of significant level are determined toaccompany said received Q-channel baseband signal, comb filtering saidreceived I-channel baseband signal to generate comb-filtered I-channelbaseband signal; if said artifacts of co-channel NTSC interference ofsignificant level are determined to accompany said received Q-channelbaseband signal, symbol decoding responsive to said comb-filteredI-channel baseband signal; and if said artifacts of co-channel NTSCinterference of significant level are determined to accompany saidreceived Q-channel baseband signal, postcoding the result of symboldecoding responsive to said comb-filtered I-channel baseband signal togenerate decoded symbols for said trellis decoding.
 3. The method ofclaim 2, further comprising steps of: if said artifacts of co-channelNTSC interference of significant level are determined to accompany saidreceived Q-channel baseband signal, conforming the comb-filteredresponse to said 1-channel baseband signal to an ideal comb filterresponse, prior to said symbol decoding responsive to said comb-filteredI-channel baseband signal; and if said artifacts of co-channel NTSCinterference of significant level are determined not to accompany saidreceived Q-channel baseband signal, equalizing said received I-channelbaseband signal without comb filtering thereof, prior to said symboldecoding responsive to said received I-channel baseband signal withoutcomb filtering thereof.
 4. A digital television signal receivercomprising: amplifier circuitry for supplying an amplifiedvestigial-sideband amplitude-modulated digital television signal apt tobe accompanied by co-channel interfering analog television signal; acomplex demodulator responsive to said amplified vestigial-sidebandamplitude-modulated digital television signal for supplying an I-channelbaseband signal containing artifacts of any co-channel interferinganalog television signal and a Q-channel baseband signal containingfurther artifacts of any co-channel interfering analog televisionsignal; symbol decoding apparatus for said I-channel baseband signalincluding a first data slicer for symbol decoding said I-channelbaseband signal during first times, errors in first symbol decodingresults from said first data slicer being correctable as long as saidartifacts of any co-channel interfering analog television signal aregenerally below a significant level for said I-channel baseband signal;and an NTSC co-channel interference detector responsive to saidQ-channel baseband signal, for detecting the presence of said furtherartifacts of any co-channel interfering analog television signal thatare above a significant level for said Q-channel baseband signal, whichsaid significant level for said Q-channel baseband signal corresponds tosaid significant level for said I-channel baseband signal.
 5. Thedigital television signal receiver of claim 4, wherein said NTSCco-channel interference detector responsive to said Q-channel basebandsignal comprises: a delay device for delaying said Q-channel basebandsignal to generate differentially delayed Q-channel baseband signals; anadder for additively combining said differentially delayed Q-channelbaseband signals to generate a sum signal; a subtractor fordifferentially combining said differentially delayed Q-channel basebandsignals to generate a difference signal; a first amplitude detector fordetecting the amplitude of said sum signal to generate a first amplitudedetection response; a second amplitude detector for detecting theamplitude of said difference signal to generate a second amplitudedetection response; and an amplitude comparator for comparing said firstand second amplitude detection responses and indicating said furtherartifacts of any co-channel interfering analog television signal areabove said significant level for said Q-channel baseband signal whensaid first and second amplitude detection responses differ more than aprescribed amount.
 6. The digital television signal receiver of claim 5,wherein said delay device within said NTSC co-channel interferencedetector generates said differentially delayed Q-channel basebandsignals with differential delay of twelve symbol epochs.
 7. The digitaltelevision signal receiver of claim 5, wherein said delay device withinsaid NTSC co-channel interference detector generates said differentiallydelayed Q-channel baseband signals with differential delay of six symbolepochs.
 8. The digital television signal receiver of claim 5, whereinsaid delay device within said NTSC co-channel interference detectorgenerates said differentially delayed Q-channel baseband signals withdifferential delay of 1368 symbol epochs or two NTSC video scan lines.9. The digital television signal receiver of claim 5, wherein said delaydevice within said NTSC co-channel interference detector generates saiddifferentially delayed Q-channel baseband signals with differentialdelay of 179,208 symbol epochs or 262 NTSC video scan lines.
 10. Thedigital television signal receiver of claim 5, wherein said delay devicewithin said NTSC co-channel interference detector generates saiddifferentially delayed Q-channel baseband signals with differentialdelay of 718,200 symbol epochs or two NTSC video frames.
 11. The digitaltelevision signal receiver of claim 4, wherein said NTSC co-channelinterference detector responsive to said Q-channel baseband signalcomprises: a first delay device for delaying said Q-channel basebandsignal to generate differentially delayed Q-channel baseband signals; afirst adder for additively combining said differentially delayedQ-channel baseband signals to generate a first sum signal; a firstsubtractor for differentially combining said differentially delayedQ-channel baseband signals to generate a first difference signal; asecond delay device for delaying said first sum signal to generatedifferentially delayed first sum signals; a third delay device fordelaying said first difference signal to generate differentially delayedfirst difference signals, said third delay device delaying said firstdifference signal similarly to said second delay device delaying saidfirst sum signal; a second adder for additively combining saiddifferentially delayed first sum signals to generate a second sumsignal; a second subtractor for differentially combining saiddifferentially delayed first difference signals to generate a seconddifference signal; a first amplitude detector for detecting theamplitude of said second sum signal to generate a first amplitudedetection response; a second amplitude detector for detecting theamplitude of said second difference signal to generate a secondamplitude detection response; and an amplitude comparator for comparingsaid first and second amplitude detection responses and indicating saidfurther artifacts of any co-channel interfering analog television signalare above said significant level for said Q-channel baseband signal whensaid first and second amplitude detection responses differ more than aprescribed amount.
 12. The digital television signal receiver of claim4, wherein said NTSC co-channel interference detector responsive to saidQ-channel baseband signal comprises: a first delay device for delayingsaid Q-channel baseband signal to generate differentially delayedQ-channel baseband signals; a first adder for additively combining saiddifferentially delayed Q-channel baseband signals to generate a firstsum signal; a first subtractor for differentially combining saiddifferentially delayed Q-channel baseband signals to generate a firstdifference signal; a second delay device for delaying said first sumsignal to generate differentially delayed first sum signals; a thirddelay device for delaying said first difference signal to generatedifferentially delayed first difference signals, said third delay devicedelaying said first difference signal similarly to said second delaydevice delaying said first sum signal; a second adder for additivelycombining said differentially delayed first difference signals togenerate a second sum signal; a second subtractor for differentiallycombining said differentially delayed first sum signals to generate asecond difference signal; a first amplitude detector for detecting theamplitude of said second sum signal to generate a first amplitudedetection response; a second amplitude detector for detecting theamplitude of said second difference signal to generate a secondamplitude detection response; and an amplitude comparator for comparingsaid first and second amplitude detection responses and indicating saidfurther artifacts of any co-channel interfering analog television signalare above said significant level for said Q-channel baseband signal whensaid first and second amplitude detection responses differ more than aprescribed amount.
 13. The digital television signal receiver of claim4, further comprising: data synchronization circuitry for determiningwhen symbols used for data synchronization appear in said I-channelbaseband signal; and circuitry for generating ideal symbol decodingresults when symbols used for data synchronization are determined toappear in said I-channel baseband signal; wherein said symbol codingapparatus further comprises: a first delay device for exhibiting a delayof a prescribed first number of said symbol epochs, connected to respondto said I-channel baseband signal, thereby to generate differentiallydelayed I-channel baseband signals; a first linear combiner whichlinearly combines said differentially delayed I-channel basebandsignals, to generate a first comb filter response in which saidartifacts of any co-channel interfering analog television signal aresuppressed; a second data slicer for symbol decoding said first combfilter response, for generating first precoded symbol decoding results;a second linear combiner which linearly combines respective first andsecond input signals received thereby for supplying a respective outputsignal therefrom as a second comb filter response, said second linearcombiner connected to receive said first precoded symbol decodingresults as said respective first input signal thereof, one of said firstand said second linear combiners being an adder and the other of saidfirst and said second linear combiners being a subtractor; a seconddelay device connected for delaying a respective input signal thereofsaid prescribed first number of symbol epochs to generate said secondinput signal of said second linear combiner; a plural-input firstmultiplexer connected for supplying a respective output signal therefromto said second delay device as said second input signal thereof, forreceiving said ideal symbol decoding results as a first of its inputsignals, for receiving said interim symbol decoding results as a secondof its input signals and for receiving said output signal of said secondlinear combiner as a third of its input signals, said first multiplexerbeing conditioned to reproduce as its output signal the first of itsinput signals when and only when symbols used for data synchronizationare determined to appear in said I-channel baseband signal, said firstmultiplexer otherwise being conditioned to reproduce as its outputsignal the output signal of said second linear combiner when said NTSCco-channel interference detector detects the presence of said furtherartifacts of any co-channel interfering analog television signal beingabove said significant level for said Q-channel baseband signal, andsaid first multiplexer otherwise being conditioned to reproduce as itsoutput signal the output signal of said first data slicer when said NTSCco-channel interference detector does not detect the presence of saidfurther artifacts of any co-channel interfering analog television signalbeing above said significant level for said Q-channel baseband signal.14. The digital television signal receiver of claim 13, wherein saidNTSC co-channel interference detector responsive to said Q-channelbaseband signal comprises: a third delay device for delaying saidQ-channel baseband signal said prescribed first number of symbol epochsto generate differentially delayed Q-channel baseband signals; a thirdlinear combiner which linearly combines said differentially delayedQ-channel baseband signals to generate a third comb filter response inwhich artifacts of any co-channel interfering analog television signaltend to be suppressed; a fourth linear combiner which linearly combinessaid differentially delayed Q-channel baseband signals to generate afourth comb filter response in which artifacts of any co-channelinterfering analog television signal tend to be reinforced one of saidthird and said fourth linear combiners being an adder and the other ofsaid third and said fourth linear combiners being a subtractor; a firstamplitude detector for detecting the amplitude of said third comb filterresponse to generate a first amplitude detection response; a secondamplitude detector for detecting the amplitude of said fourth combfilter response to generate a second amplitude detection response; andan amplitude comparator for comparing said first and second amplitudedetection responses and indicating said further artifacts of anyco-channel interfering analog television signal are above saidsignificant level for said Q-channel baseband signal when said first andsecond amplitude detection responses differ more than a prescribedamount.
 15. The digital television signal receiver of claim 14; whereinsaid first, second and third delay devices each provide differentialdelay of twelve symbol epochs; wherein said first and third linearcombiners are subtractors; and wherein said second and fourth linearcombiners are adders.
 16. The digital television signal receiver ofclaim 14; wherein said first, second and third delay devices eachprovide differential delay of six symbol epochs; wherein said first andthird linear combiners are adders; and wherein said second and fourthlinear combiners are subtractors.
 17. The digital television signalreceiver of claim 14; wherein said first, second and third delay deviceseach provide differential delay of 1368 symbol epochs or two NTSC videoscan lines; wherein said first and third linear combiners are adders;and wherein said second and fourth linear combiners are subtractors. 18.The digital television signal receiver of claim 14; wherein said first,second and third delay devices each provide differential delay of179,208 symbol epochs or 262 NTSC video scan lines; wherein said firstand third linear combiners are adders; and wherein said second andfourth linear combiners are subtractors.
 19. The digital televisionsignal receiver of claim 14; wherein said first, second and third delaydevices each provide differential delay of 718,200 symbol epochs or twoNTSC video frames; wherein said first and third linear combiners areadders; and wherein said second and fourth linear combiners aresubtractors.